1. Field of the Invention
The present invention relates to a solid-state image pickup element and a method of producing it, and more particularly to an improvement in characteristics of a solid-state image pickup element having a microstructure.
2. Description of the Related Art
A solid-state image pickup element using a CCD which is an imaging device such as an area sensor has a basic structure having: a photoelectric converting portion such as a photodiode; a portion for reading a charge from the photoelectric converting portion; and a charge transferring portion comprising charge transfer electrodes for transferring the read charge. The charge transfer electrodes are adjacently arranged on a charge transfer channel formed in the surface of a semiconductor substrate, and sequentially driven by a clock signal.
Recently, in solid-state image pickup elements, the number of imaging pixels is increased, and hence miniaturization of the pixels is advancing. In accordance with this, also miniaturization of a photoelectric converting portion is advancing. Therefore, it is difficult to maintain a high sensitivity.
To comply with this situation, various methods of efficiently converging light which reaches the vicinity of an opening to a photoelectric converting portion have been proposed.
For example, a technique has been disclosed in which a hole portion is formed in a planarizing layer in a position immediately above a light receiving portion, the hole portion is then filled with a high-refractive index material to form an optical waveguide, and light is totally reflected by an interface between the high-refractive index film serving as the optical waveguide and the planarizing layer, whereby light is introduced into the light receiving portion. Furthermore, a structure has been proposed in which, in a solid-state image pickup element having: a light receiving portion formed in a substrate; an optical waveguide that confines and propagates incident light within an interlayer film formed on the substrate, to guide the light to the light receiving portion, and a gap is formed between the optical waveguide and the interlayer film, thereby further enhancing the light guiding function.
As an example of a solid-state image pickup element having such an optical waveguide structure, in order to further enhance the light converging property, a structure has been proposed in which, as shown in FIG. 10, an insulating layer 23I made of silicon oxide or the like in a photodiode is etched to form a hole for an optical waveguide, and a reflection film M configured by a thin metal film is formed on the sidewall of the hole (JP-A-7-45805). The hole is filled with a transparent material such as a silicon oxide film to form an optical waveguide.
In the solid-state image pickup element, the optical waveguide is formed on the surface of a semiconductor substrate in which a photodiode (photoelectric converting portion) 30 and a charge transferring portion 40 are formed, and the reflection film M configured by a thin metal film is formed on the sidewall of the optical waveguide.
In this structure, the opening of the optical waveguide is narrow, and therefore it is difficult to guide obliquely incident light to the photodiode.
Therefore, a structure has been proposed in which, in a solid-state image pickup element, an opening of an optical waveguide is configured by a tapered face to be widened as shown in FIG. 11 in order to efficiently guide incident light to a light receiving portion without lowering the sensitivity of the light receiving portion, and with reducing influences due to positional displacement or optical path misalignment at a small F value or the like (JP-A-2002-118245).
In this structure, the opening of the optical waveguide is widened. In the case where the tapered face is formed in an opening portion, however, also the inclined side face is etched away when anisotropic etching is performed so that the reflection film M is selectively left on the tapered face. Therefore, it is difficult to form the thin metal film on the tapered face with high controllability.
FIGS. 12A to 12C and 13A to 13C are diagrams showing steps of producing the solid-state image pickup element shown in FIG. 10, and FIGS. 14A to 14C and 15A to 15C are diagrams showing steps of producing the solid-state image pickup element shown in FIG. 11. The steps correspond to those of Embodiment 1 which will be described later, respectively.
As described above, in the case of JP-A-7-45805 (FIG. 10) in which the side face of the optical waveguide is perpendicular to the surface of the substrate, the opening of the optical waveguide is narrow, and it is difficult to guide obliquely incident light to the photodiode. Therefore, there is a problem in that a sufficient light converging efficiency cannot be obtained. In the case of the structure of JP-A-2002-118245 (FIG. 11) in which the tapered face is formed in the opening portion, the opening of the optical waveguide can be widened, but also the inclined side face is etched away when anisotropic etching is performed so that the reflection film M is selectively left on the tapered face. Therefore, it is difficult to form a thin metal film with high controllability.
Moreover, oblique light cannot be sufficiently converged, and the photoelectric conversion efficiency may be lowered. Light from an adjacent pixel may enter to cause color mixture.